Synthesis and electrochemical properties of porphycene-diketopyrrolopyrrole conjugates

Article abstract of DOI:10.1021/ol5014608

The selective iodination of 2,7,12,17-tetrahexylporphycene 1 was successfully accomplished by using N-iodosuccinimide in the presence of activators to give 3-iodoporphycene 2 and 3,13-diiodoporphycene 3a. These iodinated porphycenes can be used as the substrates for palladium-catalyzed coupling to prepare porphycene-diketopyrrolopyrrole conjugates in two steps. The connection of the diketopyrrolopyrrole units to porphycenes broadened their absorption spectra and increased the intensity of the Q-bands due to the electronic interactions between the porphycene and diketopyrrolopyrrole moieties.

Full text of DOI:10.1021/ol5014608

Letter
pubs.acs.org/OrgLett
Synthesis and Electrochemical Properties of Porphycene−
Diketopyrrolopyrrole Conjugates
Takuya Okabe,† Daiki Kuzuhara,*,† Mitsuharu Suzuki,† Naoki Aratani,† and Hiroko Yamada*,†,‡
†Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5, Takayama-cho, Ikoma 630-0192, Japan
‡CREST, JST, Saitama 102-0076, Japan
S
* Supporting Information
ABSTRACT: The selective iodination of 2,7,12,17-tetrahexylporphycene 1 was successfully accomplished by using N-
iodosuccinimide in the presence of activators to give 3-iodoporphycene 2 and 3,13-diiodoporphycene 3a. These iodinated
porphycenes can be used as the substrates for palladium-catalyzed coupling to prepare porphycene−diketopyrrolopyrrole
conjugates in two steps. The connection of the diketopyrrolopyrrole units to porphycenes broadened their absorption spectra
and increased the intensity of the Q-bands due to the electronic interactions between the porphycene and diketopyrrolopyrrole
moieties.
orphycene, which consists of two bipyrrole units and two
coupling reaction of porphycenes. We have designed three
1
Pethylene bridges, is a constitutional isomer of porphyrin. porphycene−DPP conjugates; P−D, P−D−P, and D−P−D,
where P is porphycene and D is DPP, with ethynyl linkages at the
3- or 3,13-positions of the porphycenes (Figure 1). The
absorption of the DPP unit appears at around 450−550 nm,
while porphycene has an absorption valley around 400−550 nm.
The ethynyl linkages have increased the planarity of the
molecules and thus also enhanced the electronic conjugation
with attached functional groups. Therefore, the combination of
DPP and porphycene via ethynyl linkages is expected to generate
absorption covering the visible to NIR regions of the spectrum.
Along this line, DPP-linked porphyrin derivatives exhibit good
absorption abilities in the UV−vis−NIR range and have
functioned as the excellent p-type semiconducting materials in
an organic solar cell (OSC) with a power-conversion efficiency of
over 7%.15 In addition, DPP-linked bisporphyrin shows strong
two-photon absorption.16
Porphycene has a lower LUMO level and smaller HOMO−
LUMO difference than porphyrin since its molecular symmetry
is lower than that of porphyrin.2 These properties have allowed
us to explore the possible applications of porphycenes in the
fields of photodynamic therapy (PDT),3 photoinactivation of
viruses and bacteria,4 protein mimicry,5 nonlinear optics,6 and
catalysis.7 We recently reported solution-processed organic solar
cells, based on tetrabenzoporphycene8 as the p-layer and [6,6]-
phenyl-C61-butyric acid methyl ester (PC61BM) as the n-layer,
with a power-conversion efficiency (PCE) of 1.54%.9 The feature
of the lower LUMO level of porphycene is suitable for use in
electron-accepting materials. Jux, Guldi and co-workers have
reported porphycene derivatives which function as electron
acceptors in the composites with graphene oxide or nano-
graphene.10 Porphycenes therefore have potential as organic
semiconducting materials exhibiting unique electronic proper-
ties, even though such materials have seldom been reported. To
make a variety of porphycene derivatives, it is necessary to
develop synthetic substrates for versatile reactions. In porphyrin
chemistry, various cross-coupling reactions have led porphyrins
to various functional materials;11−14 however, there have been no
reports of the palladium-catalyzed cross-coupling reactions of
porphycenes.
We initially investigated the reaction conditions necessary for
the iodination of porphycene. Hisaeda et al. reported
bromination of 2,7,12,17-tetraalkyl-porphycene by bromine.17
By controlling the ratio of equivalents of bromine to porphycene,
the mono-, di-, tri-, and tetra-brominated porphycenes were
prepared. Waluk et al. also demonstrated the preparation of
2,7,12,17-tetra-tert-butyl-3,13-diiodoporphycene, using N-iodo-
Here we report the preparation of porphycene−diketopyrro-
lopyrrole (DPP) conjugates by the selective iodination and cross-
Received: May 20, 2014
© XXXX American Chemical Society
A
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Letter
large iodonium cation prevents the multi-iodination reaction.
The activation of the iodonium cation by TFA19 or silica gel,20
which are well-known efficient activators of NIS and NBS, was
then attempted. By addition of an excess amount of TFA with 1.1
equiv of NIS, 2 was predominantly obtained in 66% yield (entry
2), while the reaction of 1 and 2.5 equiv of NIS in the presence of
TFA gave a mixture of 3a and 3b in 35% and 12% yields,
respectively. Interestingly, the yield of 3a was increased to 54%
by using silica gel as the activator. In addition, the product ratio of
3a to 3b varied from 35/12 when using TFA to 54/13 when
using silica gel. The positions of the iodine atoms in 3a were
unambiguously elucidated by crystal X-ray diffraction analysis
(Figure S2).
We then attempted to synthesize ethynyl-linked porphycene−
DPP conjugates, applying the reaction procedure as shown in
Scheme 1. The trimethylsilyl (TMS) ethynyl group was
Scheme 1. Synthesis of Porphycene−DPP Conjugates P−D,
P−D−P, and D−P−D
Figure 1. Structures of porphycene−DPP conjugates P−D, P−D−P,
and D−P−D.
succinimide (NIS) as an iodonium cation source.18 This reaction,
however, gave a mixture of 3,13-diiodoporphycene and 3-bromo-
13-iodoporphycene because the used NIS reagent included a
small amount of N-bromosuccinimide (NBS) as a contaminant.
We attempted to optimize the iodination conditions by
referring to Waluk’s conditions (Table 1). The reaction of
Table 1. Synthetic Conditions for Iodinated Porphycenes
yield (%)
c
d
entry
NIS (equiv)
activator
2
3a
12
3b
a
1
2
3
4
5
6
1.1
1.1
1.1
2.5
2.5
2.5
none
36
3
a
introduced to 2 and 3a by Stille coupling with tributyl(trimethyl-
silylethynyl)tin to give 4 and 6 in 89% and 88% yields,
respectively. The structure of 6 was elucidated by crystal X-ray
diffraction analysis, as presented in Figure S3. When the reaction
of 3a was instead attempted by Sonogashira coupling with
trimethylsilylacetylene, the yield of 6 was only 6% and
porphycenic enyne products were obtained after purification
by GPC (see Supporting Information, Figure S4).21
TFA
66
trace
20
7
trace
5
b
silica gel
29
a
none
42
1
a
TFA
trace
trace
35
54
12
13
b
silica gel
a
b
c
In THF. In CH2Cl2. Purification by recrystallization from CHCl3/
d
hexanes three times. 3b yields were determined by 1H NMR
spectroscopy.
Subsequently, deprotection of the TMS groups of 4 and 6 by
tetrabutylammonium fluoride (TBAF) gave 5 and 7, respectively.
The porphycene−DPP dyad (P−D) was prepared from 5 by
Sonogashira coupling with monobrominated dithienyl-DPP 8 in
55% yield. The porphycene−DPP triads, P−D−P and D−P−D,
were synthesized by the same procedures: P−D−P from 5 and 9
in 52% yield and D−P−D from 7 and 8 in 75% yield,
respectively. These three obtained porphycene−DPP conjugates
were characterized by 1H NMR spectroscopy and high-
resolution matrix assisted laser desorption/ionization time-of-
flight (HR MALDI-TOF) mass spectrometry. The thermal
stabilities of P−D, P−D−P, and D−P−D were measured by
2,7,12,17-tetrahexylporphycene 1 and NIS (1.1 equiv) in THF
gave a mixture of 3-monoiodoporphycene 2 in 36% yield and
diiodoporphycenes 3a and 3b (entry 1) (Figure S1). Fortunately,
3,16-diiodoporphycene 3b could be removed by repeated
recrystallizations from chloroform/hexane to give pure 3,13-
diiodoporphycene 3a. Even though the amount of NIS was
increased to 2.5 equiv, the porphycene 1 was not completely
consumed nor were tri- and tetra-iodinated porphycenes
obtained (entry 4). These results suggested that the low
reactivity induced by the steric hindrance associated with the
B
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Letter
thermogravimetric analysis (TGA) (Figure S5). P−D showed
the onset of melting point at 184.3 °C, while P−D−P and D−P−
D exhibited weight loss starting above 300 °C, indicating that
these conjugates possess highly thermal stability.
The UV−vis absorption spectra of P−D, P−D−P, and D−P−
D in CHCl3 are shown in Figure 2 and summarized in Table 2.
indicate that electronic interactions occur between the DPP and
porphycene moieties.
The electronic properties of the three porphycene−DPP
conjugates and the reference compounds were studied by cyclic
voltammetry and differential pulse voltammetry in o-dichlor-
obenzene/acetonitrile (5:1) containing 0.1 M nBu4NPF6 at room
temperature (Figure 3). The reference compound 1 exhibits
Figure 2. UV−vis absorption spectra of P−D (black solid line), P−D−P
(blue solid line), D−P−D (red solid line), 1 (green dashed line), and
DTh−DPP (pink dashed line) in CHCl3.
Table 2. Optical and Electrochemical Properties of P−D, P−
a
D−P, and D−P−D
b
λabs (nm) (log ε)
Eox (V)
Ered (V)
P−D
373 (5.06), 550 (4.60),
586 (4.79), 647 (4.87)
0.43,
0.66,
0.84
−1.30, −1.52,
−1.75
Figure 3. Cyclic voltammograms (solid lines) and differential pulse
voltammograms (thin lines) of P−D (black), P−D−P (blue), D−P−D
(red), 1 (green), and DTh−DPP (pink) in o-dichlorobenzene/
acetonitrile (5:1) with 0.1 M nBu4NPF6. Scan rate = 100 mV s−1.
c
P−D−P
374 (5.38), 603 (4.98),
653 (5.25)
0.35,
0.49,
0.66
−1.30, −1.51,
−1.56, −1.80
D−P−D
374 (5.04), 393 (4.96),
552 (4.83), 601 (4.94),
656 (5.15)
0.41,
0.53,
0.77
−1.17, −1.32,
−1.68
reversible peaks at 0.47 V (vs ferrocene/ferrocenium cation) for
oxidation and −1.45 V for reduction. DTh−DPP shows
reversible oxidation and reduction peaks at 0.47 and −1.70 V,
respectively, while P−D has three reversible oxidation peaks
(0.43, 0.66, and 0.84 V) and three reduction peaks (−1.30,
−1.52, and −1.75 V).
a
Potential values were measured by cyclic voltammetry in dichlor-
n
obenzene/acetonitrile (5:1) with 0.1 M Bu4NPF6. The ferrocene/
ferrocenium cation redox couple was used as the internal standard.
Scan rate = 100 mV s−1. [P−D−P and D−P−D] = saturated, [P−D]
= 0.5 mM, and [1 and DTh−DPP] = 1.0 mM. Working electrode:
glassy carbon. Counter electrode: Pt wire. Reference electrode: Ag/
In order to further elucidate the structure and electronic
properties of these compounds, DFT calculations were
performed using the Gaussian 09 program package at the
B3LYP/6-31G* level.22 The structures of P−D, D−P−D, and
P−D−P were simplified by substituting methyl groups for the
hexyl and 2-ethylhexyl groups in the molecules. The results
demonstrated that the HOMO of the dyad P−D is distributed
over the DPP moiety, while the LUMO is distributed over the
porphycene core. Similarly, the orbitals of the D−P−D and P−
D−P triads are primarily situated on the DPP portion in the case
of the HOMO and at the porphycene in the case of the LUMO.
Considering the redox potentials and DFT calculations results
for the porphycene−DPP conjugates and reference compounds,
the first oxidation and reduction peaks of the conjugates are
assigned to the DPP and porphycene portions, respectively. This
result indicates that porphycene behaves as an electron acceptor,
while DPP is the electron donor in the porphycene−DPP
conjugate systems. P−D−P and D−P−D also exhibit redox
properties based on the donor (DPP)−acceptor (porphycene)
systems.
b
c
AgNO3. In CHCl3. The oxidation and reduction potentials were
determined by DPV.
The absorption spectra of 1 and dithienyl−DPP (DTh−DPP) in
CHCl3 are also provided in Figure 2 for reference purposes. The
absorption of 1 exhibits a typically intense Soret-like band and
broad Q-like bands with moderate intensity. The absorption
peak of DTh−DPP is observed at 548 nm. The maximum
absorption peaks of the porphycene−DPP conjugates P−D, D−
P−D, and P−D−P all show the Soret-like and Q-like bands in
similar wavelength regions similar to those observed for 1 and the
absorption edges of D−P−D and P−D−P extend to 750 nm.
The molar absorption coefficient of P−D−P at the Soret band is
almost twice as high as those of 1, P−D, and D−P−D because
P−D−P contains two porphycene moieties in each molecule.
The intensities of the Soret bands of P−D and P−D−P are
similar to that of 1. Interestingly, the intensities of the Q-bands
are significantly increased for all three porphycene−DPP
conjugates compared to the parent porphycene 1. In particular,
the molar absorption coefficient of P−D−P exhibits a maximum
of 178 000 M−1 cm−1 at 653 nm. In addition, the absorption of
the DPP components of all three of these compounds are not
clearly evident within the range of 500−600 nm. These results
In summary, we were successful in achieving the selective
iodination of porphycene to give 3-iodo- and 3,13-diiodo-
porphycenes for the first time. Activators such as TFA and silica
gel were found to be important with regard to improving the
yield and selectivity of these reactions. The synthesis of these
C
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Organic Letters
Letter
C.; Che, C.-M.; Cheng, K.-F.; Mak, T. C. W. Chem. Commun. 1997,
1205.
iodoporphycenes allowed us to carry out palladium-catalyzed
cross-couplings to prepare the three porphycene−DPP con-
jugates P−D, D−P−D, and P−D−P. The introduction of DPP
units through ethynyl linkages to porphycene resulted in
broadened absorption bands in the visible to NIR regions and
enhancement of the molar absorption coefficients of the Q-band
regions. The electrochemical properties of these porphycene−
DPP conjugates showed that multiredox properties were
obtained based on the donor−acceptor structures of the
molecules. The optical and electrochemical properties of these
porphycene−DPP conjugates are expected to allow these
compounds to exhibit superior performance as the OSC
materials. Extension of this synthetic strategy by the palladium-
catalyzed cross-couplings of 3,13-diiodoporphycene and appli-
cations of the resulting porphycene−DPP conjugates in the OSC
devices are currently underway.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental and computational details, and characterization
data for the new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
(13) (a) Luo, J.; Xu, M.; Li, R.; Huang, K.-W.; Jiang, C.; Qi, Q.; Zeng,
W.; Zhang, J.; Chi, C.; Wang, P.; Wu, J. J. Am. Chem. Soc. 2014, 136, 265.
(b) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.;
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AUTHOR INFORMATION
■
Yeh, C.-Y.; Diau, E. W.-G.; Gratzel, M. Angew. Chem., Int. Ed. 2010, 49,
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Corresponding Authors
6646.
(14) Aratani, N.; Kim, D.; Osuka, A. Chem.Asian J. 2009, 4, 1172 and
*E-mail: kuzuhara@ms.naist.jp.
*E-mail: hyamada@ms.naist.jp.
Notes
references cited therein.
(15) (a) Qin, H.; Li, L.; Guo, F.; Su, S.; Peng, J.; Cao, Y.; Peng, X.
Energy Environ. Sci. 2014, 7, 1397. (b) Li, L.; Huang, Y.; Peng, J.; Cao, Y.;
Peng, X. J. Mater. Chem. A 2013, 1, 2144.
The authors declare no competing financial interest.
(16) Nowak-Krol
Wicks, G.; Chotkowski, M.; Rebane, A.; Gor
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ACKNOWLEDGMENTS
■
(17) Shimakoshi, H.; Baba, T.; Iseki, Y.; Aritome, I.; Endo, A.; Adachi,
C.; Hisaeda, Y. Chem. Commun. 2008, 2882.
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This work was supported by a Grant-in-Aid (No. 25288092 to
H.Y.), the Green Photonics Project in NAIST, and the Program
for Promoting the Enhancement of Research Universities in
NAIST sponsored by the Ministry of Education, Culture, Sports,
Science and Technology, Japan. We also acknowledge Nippon
Synthetic Chem. Ind. (Osaka, Japan) for a gift of ethyl
isocyanoacetate, which was used for the preparation of the
starting pyrroles.
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